Grouping.cs

namespace StandardOperators;

public static class Grouping
{
    public static void GroupingQueryExpression()
    {
        IEnumerable<IGrouping<string, Course>> subjectGroups =
            from course in Course.Catalog
            group course by course.Category;

        foreach (IGrouping<string, Course> group in subjectGroups)
        {
            Console.WriteLine($"Category: {group.Key}");
            Console.WriteLine();

            foreach (Course course in group)
            {
                Console.WriteLine(course.Title);
            }
            Console.WriteLine();
        }
    }

    public static void ExpandingSimpleGroupingQuery()
    {
        IEnumerable<IGrouping<string, Course>> subjectGroups = 
            Course.Catalog.GroupBy(course => course.Category);
    }

    public static void GroupingQueryItemProjection()
    {
        IEnumerable<IGrouping<string, string>> subjectGroups =
            from course in Course.Catalog
            group course.Title by course.Category;
    }

    public static void ExpandedGroupingQueryItemProjection()
    {
        IEnumerable<IGrouping<string, string>> subjectGroups = Course.Catalog
            .GroupBy(course => course.Category, course => course.Title);
    }

    public static void GroupingQueryGroupProjection()
    {
        IEnumerable<string> subjectGroups =
           from course in Course.Catalog
           group course by course.Category into category
           select $"Category '{category.Key}' contains {category.Count()} courses";
    }

    public static void ExpandedGroupingQueryGroupProjection()
    {
        IEnumerable<string> subjectGroups = Course.Catalog
            .GroupBy(course => course.Category)
            .Select(category =>
               $"Category '{category.Key}' contains {category.Count()} courses");
    }

    public static void GroupByWithKeyAndGroupProjection()
    {
        IEnumerable<string> subjectGroups = Course.Catalog.GroupBy(
            course => course.Category,
            (category, courses) =>
               $"Category '{category}' contains {courses.Count()} courses");
    }

    public static void GroupByWithKeyItemAndGroupProjection()
    {
        IEnumerable<string> subjectGroups = Course.Catalog.GroupBy(
            course => course.Category,
            course => course.Title,
            (category, titles) =>
                 $"Category '{category}' contains {titles.Count()} courses: " +
                     string.Join(", ", titles));
    }

    public static void CompositeGroupKey()
    {
        var bySubjectAndYear =
            from course in Course.Catalog
            group course by new { course.Category, course.PublicationDate.Year };
        foreach (var group in bySubjectAndYear)
        {
            Console.WriteLine($"{group.Key.Category} ({group.Key.Year})");
            foreach (Course course in group)
            {
                Console.WriteLine(course.Title);
            }
        }
    }
}